In spite of an already rich and diverse fish genetic resource of India, more than 300 exotic species have been introduced into the country so far (Jhingran, 1989a). While a vast majority of them are ornamental fishes which remain, more or less, confined to the aquaria, some others have been introduced in aquaculture and open water systems with varying degrees of success. Three larvicidal fishes viz., Lebistes reticulatus, Nothobranchus sp. and Gambusia affinis were introduced for containing the insect larvae in confined waters. Silver carp and the three varieties of common carp were brought into the country with the objectives of broadening the species spectrum in aquaculture and increasing the yields through better utilization of trophic niches. In recent years, the bighead carp Hypophthalmichthys nobilis and O. niloticus have been reported from the culture systems of eastern India. After unauthorised introduction, these two fishes are becoming popular among the aquaculturists of the region. While a few of the introduced species proved to be a boon in aquaculture and acted as an instrument for yield optimisation from ponds, the accidental and deliberate introduction of some of the exotic fishes into the open-waters has generated a lot of debate in recent years. There is a growing concern in India about the possible deleterious impact of the exotic fishes on the fish species diversity of the Indian rivers.
Oreochromis mossambicus, Hypophthalmichthys molitrix, Ctenopharyngodon idella, Cyprinus carpio communis, C. carpio specularis and C. carpio nudus have gained entry into the reservoir ecosystem through accidental or deliberate introduction. Among them, tilapia, silver carp and common carp could make a negative impact on the fisheries in various reservoirs in the country. Instances of Gambusia affinis getting naturalised in reservoirs are rare. In Markonahalli reservoir, the fish has established itself as a breeding population and reported to be affecting the larval stages of commercially important fishes.
The tilapia, O. mossambicus was first introduced into the pond ecosystem of the country in 1952 and soon it was stocked in the reservoirs of south India. By the end of 1960s, most of the reservoirs in Tamil Nadu and those in the Palakkad and Trissur districts of Kerala were regularly stocked with tilapia. Performance of tilapia in ponds of south India has been discouraging mainly due to its early maturity, continuous breeding, over-population and dwarfing. It is reported to mature at 6 cm length at an age of 75 days and to breed at an interval of one month under the tropical conditions.
Performance of tilapia in reservoirs
The warm waters of the tropical reservoirs in India have provided a conducive habitat for the tilapia and it has established a secure position in a number of south Indian reservoirs. The fears of its stunted growth have been allayed as the average size of tilapia did not decline as much as it did in ponds. Sreenivasan (1967) stated that the fluctuating water levels affected the breeding pits of the fishes and the predators took a heavy toll of their young ones. These two factors are believed to keep check on the excessive proliferation of tilapia in reservoirs.
Size of tilapia in the commercial catches of reservoirs has been very good, as opposed to the unmarketable size reported fromthe ponds. The average size of tilapia from Tamil Nadu reservoirs has been 1.5 kg during the 1960s, with the minimum size of 500 g. Similarly, tilapia weighing 2.5 kg was very common in Malampuzha reservoir, Kerala during the 1960s, with an average size of 1.5 to 1.75 kg. The present size of 0.5 to 0.7 kg in Malampuzha and 0.68 kg in Tamil Nadu reservoirs are well within the limits of market preference, their continuous slide in size over the years is a cause of concern as it is feared that if the fall in size continues, it may become unmarketable.
Tilapia has dominated and virtually eliminated all other fishes including the stocked Gangetic carps in a number of reservoirs in Tamil Nadu. Vaigai, Krishnagiri, Amaravathy, Uppar and Pambar reservoirs in Tamil Nadu are harbouring sizeable populations of tilapia since 1960s, contributing substantially to commercial catches. While its contribution has declined since 1979–80 in Vaigai, it continues to form a major fishery in all the other reservoirs. In Krishnagiri, the fish has a changing fortune on account of competition with the mullet, Rhinomugil corsula. From the predominant position in the 1960s the percentage of tilapia came down to 4.3% in 1983–86, only to increase in the year 1989–90 to 69% (Jhingran, 1991). At present, tilapia forms 24% of the catch.
Chulliar, Meenkara, Peechi and Malampuzha reservoirs in Kerala have been stocked with tilapia in the early sixties and the fish contributes substantially to the catch. In the relatively large (2 313 ha) Malampuzha reservoir, tilapia is reported to have registered a fast growth rate and large size, contributing 10 to 70% of the catch in different years. Tilapia also found its way into Kolleru lake in Andhra Pradesh and Sondur reservoir in Raipur district, Madhya Pradesh. Jhingran (1991) examined the yield of tilapia in different classes of reservoirs in the size range 50 to 10 000 ha and came to the conclusion that, by and large, small reservoirs in the size range of 50 to 200 ha showed a better yield (44 to 101 kg ha-1), than the larger ones. Relatively large reservoirs such as Malampuzha and Amaravathy have a better size of tilapia, although documentation is not sufficient to attempt any correlation between the size attained by the fish and the area of reservoirs.
Distribution of tilapia is more or less restricted to the tropical belt as the fish is constrained with slow growth and winter mortalities in the higher latitude. Attempts to introduce the fish in Baghla, a small irrigation impoundment in the Gangetic plain have not succeeded (Jhingran, 1991). The ongoing debate on the introduction of tilapia centres round its:
suitability to enhance yield through niche utilization,
propensity for affecting or even replacing the native ichthyofauna, and
One of the main considerations in determining introduction of exotic fishes is their feeding habits. Since none of the Indian culturable carps feeds on Cyanophyceae blooms like Microcystis aeruginosa, tilapia is often cited as a welcome addition to the blue-greens-dominated water bodies. Although Sreenivasan (1967) and many workers abroad expressed their reservations about the ability of tilapia to digest and assimilate Microcystis, Jhingran (1991) believed that the sodium-calcium ratio in alkaline reservoirs broke down the cell walls of blue-greens, facilitating their digestion by tilapia. Oreochromis mossambicus has wider omnivorous food spectrum, compared to many other species of tilapia.
Tilapia versus indigenous carps
The apprehensions about tilapia affecting the native ichthyofauna seem to be valid, going by its track record in India and abroad. In a number of reservoirs in India, introduction of tilapia has resulted in poor growth rate or even elimination of the indigenous species and the transplanted Gangetic carps. Sreenivasan (1967) found that the growth rates of C. catla, L. fimbriatus and C. mrigala were adversely affected by tilapia in Ayyamkulam pond. He also observed that growth of Chanos chanos was restricted to less than 100 g yr-1, against the usual 500 g yr-1 in many water bodies of Tamil Nadu due to its co-existence with tilapia. In Kabini reservoir tilapia has adversely affected the indigenous Cirrhinus reba. During the period from 1980–81 to 1984–85, tilapia has caused decrease of C. reba's share in the catch from 70% to 20% (Murthy et al., 1986). Tilapia and the Indian economic species sharing a common food niche, the success of one in competition with the other is determined by the ability to breed and propagate. Given the propensities of tilapia for autostocking, the indigenous species, which are prone to breeding failure, have a definite disadvantage in its struggle to coexist with the former.
Introduction of O. mossambicus and Tilapia zillii for weed and insect control in Californian reservoirs has affected the native ichthyofauna (Moyle, 1976). A more devastating effect on indigenous fish fauna has been reported from Kyle reservoir in Zambia, where a valuable local species Paretopus petite has been eliminated from the ecosystem (Lamarque et al., 1975).
Tilapia of right size has a good consumer preference. Tilapia is also known as a species affordable by the poor. In Palakkad and Trissur districts of Kerala, where the fish is an important component of reservoir catch, tilapia enjoys a consumer preference over the Indian carps even when sold at an equal price (Jhingran, 1991). Relevance of tilapia in reservoir fisheries
Tilapia (O. mossambicus) has entered the Indian scene, when the inland fisheries contributed negligibly to the total fish production in the country and the ecosystem management was in its infancy. Today, a number of indigenous species are available for stocking to broaden the species spectrum, bridge the gaps in niche utilization and increase the yield. Barring a very few reservoirs, tilapia-dominated fishery invariably leads to low yields. In many reservoirs like Krishnagiri and Vaigai, the production has been found to be erratic due to the unpredictive behaviour of tilapia population due to competition from other fishes. Fishery managers of India are striving hard to change the fish from its dominant position, wherever, they occupy one. Thus, in the present context, tilapia does not figure among the species preferred for stocking in Indian reservoirs.
Oreochromis niloticus has not yet entered the reservoir ecosystem in India. Confined to the estuarine and freshwater wetlands of the eastern India, the fish has registered an impressive growth of 250 g in 6 months. Since this fish is not reported to have problems of stunted growth and prolific breeding, it may probably have a more positive role to play in Indian reservoirs, compared to O. mossambicus.
Silver carp, Hypophthalmichthys molitrix was introduced in India in 1959 and unlike tilapia, it has not strayed into many reservoirs. However, silver carp has attracted more attention from the ecologists and fishery managers, generating a more animated debate. Importance of silver carp in reservoirs emanates mainly from:
its reported ability to utilise Microcystis
the impressive growth rate, and
its propensities for affecting the indigenous species, especially Catla catla.
The silver carp has a specialised structure of gill rakers adapted to microplankton feeding (Inaba and Nomura, 1956). Gut analysis of the fish carried out at the Central Inland Fisheries Research Institute revealed a wide feeding range including Chlorophyceae, Cyanophyceae, Chrysophyceae, Bacillariophyceae, Dinophyceae, Protozoa, Rotifera, Cladocera, Ostracoda, and Copepoda (Jhingran and Natarajan, 1978).
Performance of silver carp in reservoirs
An experimental consignment of 239 fingerlings of silver carp was stocked in Kulgarhi reservoir (Madhya Pradesh) in 1969. Based on the recapture of 8 specimens, growth rates ranging from 597 mm in 783 days (0.76 mm day-1) to 404 mm in 293 days (1.4 mm day-1) have been recorded (Rao and Dwivedi, 1972). The fish was also introduced in Getalsud reservoir, Bihar in 1974, where it has recorded growth rates ranging from 2.20 to 5.79 g day-1 (Table 13.4). In both reservoirs, the fish did not breed. There are reports about the stocking of silver carp in a number of reservoirs across the country ranging from Gumti in the northeast to Aliyar in Tamil Nadu. However, it did not get established as a breeding population anywhere.
The most spectacular performance of silver carp has been reported from Gobindsagar reservoir, where after an accidental introduction, the fish formed a breeding population and brought about a phenomenal increase in fish yields. (see the chapter on Himachal Pradesh). Silver carp is instrumental in enhancing the fish production from the reservoir from 160 t in 1970–71 to 964 t in 1992–93 (Fig. 1.10). Many workers have suggested the over-intensity of feeding in respect of silver carp, as reflected by the gorged and full conditions of gut all through the year. Jhingran and Natarajan (1978) pointed out that the silver carp, being a cold-water fish, when introduced into a warm regime, consumed food much in excess and grew faster as expected of a true poikilotherm. A similar latitude- induced change worth noticing is the age at maturity. The fish mature when they are 5 to 6 years old in North China, 4 to 5 years in Central China and 2 to 3 years in south China. In India, it breeds just at the age of one year under optimum conditions.
Silver carp versus catla
Pond culture experiments conducted in the country have unmistakably established the superior performance of silver carp when cultured along with catla (Sukumaran, et al., 1968). Jhingran and Natarajan (1978) expressed the view that this need not hold good for large water bodies like reservoirs. They argued that the silver carp short-circuited the food-web, resulting in the poor performance of catla. For instance, silver carp showed preference for smaller zooplankters especially rotifers and nauplii. It was quite likely that too much of grazing on copepode nauplii by silver carp could disturb the life cycle of copepodes in a small water body like pond, causing poor performance of catla. While advocating a cautious approach, they advocated stocking of silver carp in closed reservoirs like Gobindsagar and Nagarjunasagar which are blocked by dams, both down- and upstream and no connected with the Ganga river system, the original abode of C. catla. The fish was not stocked in either of the reservoirs. However, in Gobindsagar, stocking of silver carp has since become irrelevant as the fish has already carved out a niche for itself in the reservoir. In the process, the lurking fear that the exotic fish is deleterious to the populations of indigenous catla has been proved beyond any doubt. Ever since silver carp gained a stronghold in the reservoir, catla which constituted an annual fishery of the magnitude of 200 to 300 t has declined considerably (Fig. 1.10)
Karamchandani and Mishra (1980), while evaluating the co-existence of silver carp and catla, established that the two fishes shared a common niche and compete with each other for food in a reservoir ecosystem. Percentage composition of phytoplankton in the guts of both the fishes caught during the same time from Kulgarhi reservoir was more or less the same. Zooplankton, the favourite menu of catla, formed 21% of the gut contents of silver carp. The authors concluded that silver carp hampered the growth of catla in the reservoir and advocated caution before its stocking in Indian reservoirs.
It is significant to note that despite its entry into a number of Indian reservoirs, by accident or otherwise, silver carp failed to get naturalised anywhere except Gobindsagar. Considering that the reservoir, with its temperate climate, is closer to the original habitat of the fish and has a distinctly cold water hypolimnion due to the discharge from Beas, the silver carp seems to have found a congenial habitat for growth and propagation. Although introduction of silver carp was never cleared by the Committee of Experts constituted by Govt. of India, the fish is being stocked in a number of reservoirs in the country. Nowhere did the fish make an impact as it did in Gobindsagar. Therefore, fears regarding the threat of extinction of catla from the Gangetic and peninsular India posed by silver carp are perhaps misplaced.
Figure 1.10. Catch of silver carp in relation to the total fish catch in Gobindsagar (in t)
The three varieties of the Prussian strain of common carp, viz., the scale carp (Cyprinus carpio communis), the mirror carp (C. carpio specularis) and the leather carp (C. carpio nudus) were introduced in India during 1939. They were stocked in several high altitude ponds and lakes during the 1950s. Later, in the 1957, the Chinese (Bangkok) strain of the common carp was brought into the country, primarily for aquacultural purposes, considering its warm water adaptibility, easy breeding, omnivorous feeding habits, good growth and hardy nature.
Like tilapia, common carp soon found its way to all types of reservoirs in the country. Relative ease at which the fish could breed in controlled conditions prompted the departmental fish farms throughout the country to produce the seed of common carp in large numbers and to stock them in the reservoirs. However, such stocking attempts were devoid of any ecological reasoning. The Bangkok strain of common carp has been stocked in a large number of reservoirs in the plains and European strain was introduced in the reservoirs of temperate zones and high altitudes. But their performance in reservoirs is erratic, despite heavy stocking.
Common carp is not a suitable fish for stocking in Indian reservoirs, especially the larger ones, for diverse reasons. Being a sluggish fish, its chances of survival in a predator-dominated reservoir are very poor. They are not frequently caught in a passive fishing gear like gill net, due to its slow movement and bottom dwelling habit. It is no wonder, despite a regular stocking for 13 years (involving 537 000 fingerlings), not a single common carp was ever caught from Nagarjunasagar. Obviously, the stocked fishes failed to survive among the marauding predators. This has been the fate of common carp stocked in all the deep reservoirs, with a few exceptions such as Krishnarajasagar. A more important disqualification is its propensities to complete with some important indigenous carps like Cirrhinus mrigala and C. cirrhosa and C. reba with which common carp shares food niche. Instances where the presence of common carp has resulted in the decline of Cirrhinus sp. are available in Girna and Krishnarajasagar.
The mirror carp has a dubious distinction of jeopardizing the survival of a number of native fish species, after its introduction in some of the upland lakes of Kumaon Himalayas, the Dal lake in Kashmir, Gobindsagar, and the reservoirs of the northeast. In the Dal lake, common carp found a favourable environment by virtue of the shallow lake basin, extensive submerged vegetation, and rich food resources. By virtue of the specific ecological advantage, the fish propagated itself profusely to the perli of indigenous snow trouts like Schizothoraichthys niger, S. esocinus, and S. curvifrons. The snow trouts had the twin disadvantages of low fecundity and the stream breeding behaviour. Mirror carp has caused similar damage to the snow trouts in Gobindsagar reservoir and Osteobrama belangeri in Loktak lake of the northeast. Analogy of events related to the common carp and snow trouts sends out enough signals regarding the potential harm the former can do to the ichthyofauna in the plains.
Other exotic species
Three exotic carps, being considered for introduction in the country are the bighead carp, H. nobilis (already gained entry unofficially) the mud carp, Cirrhinus molitorella and the snail carp, Mylopharyngodon piceus, feeding on zooplankton, detritus and molluscs respectively. Natarajan (1988), after a thorough probe into the ecological implications of their introduction, considered their introduction as an irrational step, as all of them infringe on the food niche of economic carp species of India and the alien species possessed all propensities for causing extinction of their native counterparts.
Fertilization of reservoirs as a means to increase water productivity through abetting plankton growth has not received much attention in India. Multiple use of the water body and the resultant conflict of interests among the various water users are the main factors that prevented the use of this management option. Surprisingly, fertilization has not been resorted to even in reservoirs which are not used for drinking water and other purposes. Documentation on fertilization of reservoirs in India is scarce. Sreenivasan and Pillai (1979) attempted to improve the plankton productivity of Vidur reservoir by the application of super phosphate with highly encouraging results. As soon as the canal sluice was closed, 500 kg super phosphate with P2O5 content of 16 to 20% was applied in the reservoir when the waterspread was 50 ha with a mean depth of 1.67 m. As an immediate result of fertilization, phosphate content of water increased from nil to 1.8 mg 1-1 and that of soil from 0.242 to 0.328%. Similar improvements in organic carbon and Kjeldal nitrogen has been reported from soil and water phases on account of fertilization. Experiments were also conducted with urea in the same reservoir.
Application of lime was tried in some upland natural lakes for amelioration of excessive CO2 and acidity at the bottom (Sreenivasan, 1971). This measures, together with the application of superphosphate in Yercaud lake, raised the pH of water from 6.2 to 7.3 and decreased the CO2 in bottom water from 38 to 6.5 mg 1-1. There was a corresponding increase in species number and biomass of plankton.
The basic objective of fertilization is to increase the plankton density and thereby accelerating primary productivity. Fertilization in Vidur reservoir resulted in a marked increase in benthic and plankton communities and doubling of the primary production rate. After two successive applications of fertilizers, significant limnological changes took place including the presence of free carbon dioxide and decrease in pH and dissolved oxygen at the bottom layer of water. The methylorange alkalinity increased from 44 to 108 mg 1-1 from the surface to bottom, indicating a high organic productivity. Phosphate fertilization triggered the tropholytic activities mineralising the organic matter and producing carbon dioxide. As a direct benefit from the fertilization, a 50% increase in fish production, along with three-fold increase in the size (average weight) of catla, rohu, mrigal, L. fimbriatus and L. calbasu were achieved.
Experiments on fertilization is in progress in the 90 ha Naktra reservoir in Madhya Pradesh, under a research project of the Central Inland Capture Fisheries Research Institute. Organic and inorganic fertilisers are being applied to improve the water and soil quality status. Artificial eutrophication as a decisive management option was tried in India for the first time in Kyrdemkulai (80 ha) and Nongmahir (70 ha) reservoirs of the northeast (Sugunan and Yadava, 1991a, b) by applying poultry manure (10 t ha-1), urea (40 kg ha-1) and single superphosphate (20 kg ha-1).
Fertilization can play a key role in many small reservoirs of India, which require correction of oligotrophic tendencies. A number of reservoirs in Madhya Pradesh, the northeast and the Western Ghats, receiving drainage from poor catchments show low productivity, necessitating artificial fertilization. Chinese experience in fertilizing the small reservoirs for increasing productivity has been reassuring (Yang et al., 1990). In Shishantou reservoir, a management strategy comprising fertilization by organic and inorganic manures and feeding resulted in phenomenal production hike from 1 500 kg ha-1 to 6 000 to 7 000 kg ha-1 during 1985 to 1989. Before fertilization, the plankton biomass in Shishantou was 1.5 mg 1-1, which was raised to 6.5 mg 1-1 through application of organic fertilizers at the rate of 6.375 t ha-1. The plankton biomass, after dropping during the peak precipitation period, picked up to 20.51 mg 1-1 during the post-rainy season months, with corresponding increase in fish production.
Ecodegradation of reservoirs has been on the increase due to the rapid pace of industrialisation, poor environment management in the catchment and a variety of other factors. Apart from the direct entry of industrial, municipal and thermal wastes, the pollution load carried by the upstream rivers is also accumulated in the reservoirs. The environmental degradation in reservoirs is caused mainly by the waste discharge from industrial, municipal and agricultural sources and the thermal power plants (Table 1.10). High rate of siltation due to poor catchment management also affects the biological productivity.
|Reservoir||Name of river||Sources of pollution|
|Getalsud||Subarnarekha||Heavy engineering, chemicals and sewage.|
|Gandhisagar||Chambal||Textile, chemicals, trade effluents from Indore, Ujjain and Kota.|
|Tungabhadra||Tungabhadra||Paper, iron and steel, rayon, chemicals and sewage.|
|G.B.Pantsagar||Rend||Thermal power plant, coal washery, chemicals.|
|Bhavanisagar||Bhavani||Viscose factory effluent.|
|Hussainsagar||Musa||Trade effluents and sewage from Hyderabad city.|
|Byramangala||Vrishabhavati||Industrial effluents and city sewage|
(Modified from Joshi, 1990)
A number of reservoirs have been selected, of late, as sites for thermal power plants due to their dual utility as perennial source of water supply and disposal point for heated effluents. Thermal power generation capacity of the country has been registering a steady growth of 8% per annum and by the turn of the century, the installed capacity is expected to reach 84 000 MW. Various thermal plants in the country are estimated to generate 10 billion m3 of hot water (40° to 5°C) and 17 million t of fly ash every year. Fly ash is known to contain heavy metals such as Zn (6%), Ba (12.2%), Cu (1.3%), As (0.02%), V(0.08%), Ti(0.02%) and Mn (0.23%), which may find their way to the nearest river stretch or a reservoir.
Rihand is a large man-made lake of 46 000 ha, into which converge cooling waters from four super thermal power plants under the public sectorviz., Singrauli (2 000 MW), Vindhyachal (2 260 MW), Anpara (3 130 MW) and Rihand (3 000 MW), besides the private sectors Renusagar thermal power plant with a capacity of 210 MW. All these power generating plants are located within a small area of 30 km2. Chandra et al (1985) reported adverse effects of heated discharge on resident aquatic organisms. They recorded mortality of fish and decrease of aquatic life within 50 km of the discharge point, owing to high temperature (46 to 52°C) of the effluent. Deposition of fly ash has been reported up to 500 m downstream of the outfall point. Cooling waters of Renusagar power plant discharged into Rihand reservoir are acidic and high in chlorides. Although an increase in water temperature is known to cause deoxygenation, a rise within reasonable limit enhances photosynthetic activities resulting in supersaturation of water with oxygen.
The main ecological consequences of a heated water discharged into the aquatic ecosystem are increase in water temperature, change in chemical composition and change in metabolism and life history of aquatic communities. The heated discharge may elevate the water temperature by 8 to 10 °C which may cause mortality of fish and fish food organisms. Temperature also exerts direct influence on toxicity. Apart from the rise in temperature, discharged waters are often altered chemically during the cooling processes. Davies (1966) showed that cooling tower discharge has lower ammonia level, higher concentration of nitrate and TDS, and lower levels of organic nitrogen, when water is abstracted from a polluted water source.
Temperature above 40 °C has been reported to negatively affect the plankton and benthic communities. Generally, fishes avoid heated effluents discharge points by swimming away to safer places. They can also withstand wide fluctuation of temperature (8 to 10 °C). However, the reproduction of fish is affected due to deposition of fly ash in the marginal areas of the river/reservoir which act as their breeding grounds. All the power plants around Rihand reservoir are located near the intermediate and lotic sectors, where the fishes are known to congregate (Desai, 1993). The most deleterious among the impacts of thermal pollution is the blanketing effect on the reservoir bed. Fly ash covers extensive areas of the bottom, blanketing off the substratum, resulting in retardation or total elimination of benthic communities. Thick mat of fly ash deposit at the bottom bed over the years may seal the nutrients away from the water phase and thereby affect productivity.
A number of reservoirs contiguous to towns and cities face threat from sewage pollution. Although from the fisheries point of view, organic loading within certain limits does not hamper the productivity, sewage load in excess can cause aseptic conditions, adversely affect the biotic communities, retard productivity and render the fish unfit for human consumption. Moreover, the problem needs to be addressed from public health and aesthetic points of view. The acute cases of hyper-eutrophication due to city sewage discharge in Hussainsagar, Mansarovar, Byramangala and Sandynulla reservoirs cause serious impediments in ecosystem management. Cases of heavy fish mortality is reported in Byramangala (Raghavan et al, 1977) and Hussainsagar (Hingorani et al, 1977).
The major adverse impact of sewage pollution can be assessed from deoxygenation, high BOD load, rapid eutrophication and accumulation of heavy metals in the environment. Sharp fall in dissolved oxygen in water puts the biotic communities under severe stress. While some species can tolerate a wide range of dissolved oxygen, many communities are highly sensitive to this parameter. As a chronic effect of oxygen depletion, some of the component populations are eliminated from the riverine community, causing far reaching changes in the trophic cycle. For instance, complete absence of zooplankton during January to August and its reappearance in September represented by Keratella sp., associated with abundance of phytoplankton like Microcystis sp. Oscillatoria sp., Hormidium sp., and Nitzschia sp., have been observed downstream of the sewage effluent outfall on the Ganga and Yamuna. The outfall area is dominated by Chironomus, followed by oligochaetes (Tubifex and Nais) in both the rivers, while areas below the outfall are characterised by the dominance of Chironomus followed by gastropods and bivalves.
Apart from affecting the organisms at lower trophic levels, intensive rate of pollution from municipal sources often causes direct fish kill, especially in small reservoirs where the problem gets aggravated due to reduced water flow rate. There are potential problems relating to the use of chlorine for disinfecting the sewage effluents for public health purposes. Owing to the increased use of synthetic detergents for domestic purposes, their incidence in the sewage effluents are on the increase. Synthetic detergents being absorbed into the body system of fish impair their growth and reproductive capacity. Detergents mixed with oil may be 60 times more toxic than the oil alone. Synergistic action of detergents with insecticides has also been recorded. Its sub-lethal concentration causes thinning and elongation of respiratory epithelial cells. Sodium lauryl sulphate is more toxic to freshwater teleosts, compared to alkyl benzene sulphonate (13-60 mg 1-1). Impact of heavy metals is discussed later on in this document.
Wastes emanating from an array of industries such as chemical plants, textile mills, heavy engineering plants, paper mills, iron and steel factories, rayons, etc., cause pollutional hazards in Indian reservoirs. Several instances of ecosystem damage and fish kill due to industrial effluents have been documented. Effluents from the Kanoria chemicals discharged into Rihand reservoir are alkaline (pH 9.2) and high in total alkalinity (4 770 mg 1-1), specific conductivity (12 816 umhos), chlorides (5 173 mg 1-1) and free chlorine (1 924 mg 1-1) (Chandra et al., 1983). These wastes have shown severe toxic effects on phyto - and zooplankton. Direct fish kills have also been reported in this reservoir due to high chlorine bearing wastes (Arora, et al, 1970). Effluents from a paper mill at Brajrajnagar are discharged into Hirakud reservoir and Sugar mill wastes finding their way to a small reservoir in Gorakhpur were the cause of complete replacement of carps by the uneconomic fishes (Natarajan, 1979b). Discharge of industrial wastes consisting of dissolved and insoluble solids, free chlorine and lime of Mettur chemical factory into the surplus water channel of Stanley reservoir is reported to have resulted in large-scale mortality of carps and catfishes in summer.
An industrial unit near Sandynulla reservoir manufacturing gelatin from animal bones and the effluents from this factory bearing high BOD and phosphate are discharged into the reservoir, which is already eutrophic on account of sewage wastes from the city of Ooty. The south India Viscose, manufacturing viscose rayon and staple fibre, discharges 16 000 m3 of waste material into the river Bhavani at Sirumughai, causing pollutional hazard in Bhavanisagar reservoir, causing increase in bicarbonate and carbon dioxide and fall in dissolved oxygen and pH along with occasional fish kills. Another synthetic fibre manufacturing unit, Harihar Polyfibre discharges wastes into the tributaries of the river Tungabhadra, resulting in ecosystem degradation and fish kills (Joshi and Sukumaran, 1987). Several major and minor industries located on the river Bhadra in the industrial town of Bhadravati in Shimoga district are the source of heavy metals discharge in the inflowing water of Tungabhadra reservoir (Singit et al., 1987). Impact of industrial pollution
Industrial effluents, though comparatively less in volume, may cause considerable harm to the aquatic environment and the biotic communities including fish and ultimately affect man through food chain. Non-biodegradable and persistent types of pollutants like heavy metals, chlorinated hydrocarbon pesticides, oil components having high boiling points and radionuclides get more concentrated at higher trophic levels through biomagnification and pose threat to human health. Industrial effluents include a wide variety of chemical toxicants and heavy metals, apart from those contributing substantially to the BOD load such as pesticides which are used in processing the raw materials in many industries. In addition to the sub-lethal chronic effects on the environment, certain direct impacts are also discernible.
Natarajan (1979b) stressed the importance of protecting the upstream zones which are biologically sensitive areas. So are the head zones (i.e., that part of the reservoir into which river flows) of the reservoirs where fish concentration is much higher. Discharge of effluents into the upstream can throw up a chemical barrier for breeding migration of economic carps, apart from causing considerable mortality to spawn and hatchlings.
Chronic effects of effluent discharge
There are two general classes of effects of pollutants on water uses. Some of the dramatic effects of toxicity, including fish kills are often well-publicised. But the other class of effluxion which involves continuous chronic sublethal degradation needs a more demanding consideration. This degradation goes unnoticed except by ecologists, taxonomists and sometimes by fishermen. It is neither dramatic nor well-publicised. No gory pictures of heaps of dead fishes or large water areas covered with oil spill or debris, but the killer masquerades in the form of reduction in the rate of reproduction by aquatic species or subtle changes in the food chain pattern on which the fish populations depend. The contaminants get accumulated in the water, soil and detritus phases of the environment and get biologically magnified, as they enter into fish tissues.
The harmful industrial and municipal effluents are as diverse as they are obnoxious. There is a diversity of harmful chemical toxicants that emanate from different industrial units, the nature of substances varying, depending on the products, production processes and the raw materials used. Similarly, the agricultural runoff carry heavy load of non-biodegradable pesticides. Domestic wastes also contain a variety of chemicals, detergents and organic load. Unfortunately, the impact of these toxicants on the biotic communities is very complex and our knowledge in this regard is grossly inadequate. It is not even possible to prescribe a precise safe limit in respect of any of the chemical pollutants. The pre- 1960 literature in this regard was based on short-term studies and used mortality as an end point. This is no longer valid, as the emphasis has now shifted to a balanced ecosystem rather than prevention of fish kills. To prevent the ecosystem from gradual degradation, we must provide criteria that will protect the entire life cycle of the desirable species as well as the food chain on which these species depend. A significant reduction in available food or reproductive success will result in a condition similar to that after a fish kill. Criteria must, therefore, be based on chronic or life cycle studies that may also permit extrapolation to untested species or toxicants.
Where multiple discharges exist, many chemical reactions may occur in the receiving water that intensify or reduce the toxic effect of the original materials. Thus, apart from the information concerning the effluents, specific knowledge on the potential chemical and physical changes involved is imperative to estimate the effects of multiple effluents on the environment. Moreover, the potential of combined stress on aquatic life cannot be explained on the basis of a single contaminant. The problems involving pH and metal toxicity are common where toxicity increases due to decrease in pH values. Similarly, the environments barely acceptable with regard to dissolved oxygen become totally unacceptable if the temperature is permanently increased, resulting in an increase in oxygen demand by the aquatic life.
The validity of applying the existing safe concentration limits is rather limited, as they are generally determined under controlled laboratory conditions. In the laboratory, fish are fed ad libitum. They are treated prophylactically, if needed: there are no predators, no competition for spawning areas, and no exposure to extremes of natural water quality. The effect of 2, 4-D on fishes adequately illustrates the gradual unspectacular decline in the quality of aquatic life. A study conducted at the Bureau of commercial fisheries pesticide laboratory at Gulf Breeze, Florida (Brungs, 1972) indicated that fish exposed to 2, 4-D for 1 to 5 months grew and survived as well as the control animals. However, the exposure, apparently lowered the general body resistance to a microsporidian parasite and a massive invasion of the central nervous system of the fish resulted.
Pesticides and heavy metals
Hazardous and toxic substances such as pesticides and heavy metals are carried to the reservoirs through the effluents and the rain washings from the catchments. These substances are highly persistent and thereby contaminate the entire biogeochemical cycle of static systems like reservoirs. The problems are aggravated due to the capacity of toxic substances to get biomagnified in fish tissues, which otherwise exist in water in extremely low concentrations. Such a situation, apart from resulting in low fish, transports toxic metals and pesticides into the human body through the contaminated fish. Heavy metal accumulation in water, sediments and plant tissue has been reported from Byramangala, (Table 1.11), Stanley (Table 2.8), Hussainsagar (Table 5.12) and Rihand (Agarwal and Kumar, 1978) reservoirs and the Tungabhadra river (Joshi and Sukumaran, 1987).
Pesticide residues have been detected in the riverine ecosystems in all the river basins of the country (Joshi, 1990). High levels of BHC, methyl parathion, endosulfan and DDT and their biomagnification in biotic communities like plankton, benthos and fish have been reported from Cauvery, Ganga and Yamuna rivers. However, such observations from reservoirs are rare. Joshi (1990) recorded significant presence of residues of different isomers of BHC, and DDT and its metabolites (DDE, DDD) in fish and plankton of Rihand reservoir. Considering that the reservoir is situated in a relatively remote place far away from agricultural and industrial activities, the observation assumes importance.
Studies conducted in Panchet reservoir, Bihar (Gopalakrishnan et al., 1966) showed adverse effects of effluents from coal washings on the recruitment of Indian major carps. Sinha (1986) reported rapid eutrophication in man-made lakes due to drainage from coal fields in south Bihar.
|Water (μg l-1||Sediment (μg 1||Plant μg g-1 dry wt.)|
(After Joshi, 1990)
Excessive siltation leading to drastic decrease in the water holding capacity and even damage to concrete hydraulic structures is a common problem in reservoirs. Siltation also hampers the productivity of water body by affecting the life processes of biotic communities. Erosion of top soil in the catchment area is the main man-made factor that leads to increased sediment load in rivers. Vegetation cover on the slopes acts as an adherent of top soil during the surface runoff. Removal of forest cover through logging, grazing, road construction or for urban needs makes the soil susceptible to erosion. The entire suspended and bed load materials carried by the rivers, however, are not exclusively the contribution by man. The catchment areas, especially those of the Ganga river are characterised by a prolonged dry season followed by a turbulent monsoon, with river discharges up to 85 000 m3 per second. Therefore, heavy erosion and high sediment load are characteristic of Indian rivers. However, the tampering of environment in catchment areas adds considerably to the sediment load and the problem needs to be addressed through appropriate conservation measures.
Suspended particles tend to settle down in the lentic waters of the reservoir causing many problems. It is estimated that in India 5 334 million t of soil is eroded every year from the cultivable land and forests. The Indian rivers carry about 2 050 million t of silt, of which nearly 480 million t is doposited in the reservoirs and 1 572 million t is washed away into the seas. Loss of storage capacity of the reservoirs due to siltation is one of the most serious consequences of soil erosion. Many of the reservoirs recorded siltation rates, much in excess of what was envisaged during the planning stage of the project, due to increased rate of sediment load in the incoming waters (Table 1.12).
|RESERVOIR||Rate of silting (in ha m 100 km-2 yr-1)|
(After, Joshi, 1990)
Apart from diminishing the water holding capacity of the reservoir and cutting its life, Siltation also affects the biota by blanketing the benthic and periphytic community. It also hampers the recruitment by destroying the breeding grounds and retards the overall productivity of the ecosystem.
The unconventional production systems, such as cage and pen cultures have not become very popular in India, although they have a definite role to play in augmenting fish production from open water, especially the reservoirs. It is now widely accepted that the pen enclosures erected in the reservoir margins can be used as nurseries to raise stocking material to obviate the necessity for constructing concrete nursery farms which are cost-intensive. Similarly, the rearing of fish in cages and pens up to marketable size enables easier stock manipulation and total harvesting. However, non-standardization of farm practices and the materials to be used in the operation still acts as a major retardant for large-scale adoption of these culture systems in Indian reservoirs.
Main criteria for the choice of candidate species for cage and pen culture are:
fast growth rate,
adaptability to the stresses in enclosures due to crowded conditions,
ready acceptance of artificial feeds consisting mainly of cheap agricultural byproducts,
high feed conversion rates,
resistance to diseases, and
good market demand.
The candidate species should preferably not breed in the cages and upset the population balance. Under the Indian conditions, the Gangetic major carps(C. catla, L. rohita, C. mrigala), the chinese carps (Hypophthalmichtys molitrix, Ctenopharyngodon idella), common carps (Cyprinus carpio), the magur (Clarias batrachus) and tilapias satisfy these requirements to a great extent. Murrels (Channa spp.) also can be cultured in maritime States, where marine trash fish is available at a discount. Selection of species, however, is mainly dictated by the local demands and availability of quality seed and other inputs in adequate quantities.
Appropriate site selection is important for successful enclosure aquaculture. Sheltered, weed-free, shallow bays are the ideal locations for installing pens and cages. The sites should have adequate circulation of water, with wind and wave action within moderate limits. Excessive turbulence may lead to wastage of fish energy for stabilizing themselves and loss of feed. The other major considerations are that the water should be pollution-free, availability of seed in the vicinity, easy accessibility to the site and a ready market for fish. Flowing waters with a slow current of 1.0 to 9.0 m per minute are considered ideal for cage siting. It is desirable to install cages a little away from the shore to prevent poaching and crab menace.
Water level fluctuation is the most important consideration in site selection for the pen culture operations in reservoirs. A scrutiny of the contour map and the monthly fluctuation patterns of reservoir levels will enable the location of suitable sites, which retain sufficient water for the required period of time. Sites which dry out during summer will be ideal, as it is easier to erect pens on dry land, to be inundated later as the water level increases. Similarly, some bays of the reservoir retaining water for sufficient period can be identified and cordoned off by erecting barricades.
Experiments on cage culture conducted in India have been exploratory in nature and the yields obtained, so far, are not impressive. The supplemental feeds given are oilcakes, ricebran, soy bean flour and silkworm pupae, which have great demand in cattle, poultry, pig rearing and other animal husbandry practices and hence command a good price in the market. The food quotient obtained in the cage culture of various species has not been high, except in the case of tilapia, making conventional supplemental feeding unremunerative. The low production and feed conversion rates are mainly due to the relatively low stocking density and many deficiencies in the feed. The feed is often not in a water–stable form and nutritionally balanced to promote growth. There is need for evolving suitable complete feeds for individual species of fish from the locally available raw materials, by experimentation.
One of the major constraints of the cage culture system is the lack of suitable cage designs to withstand severe wave action, common in Indian reservoirs. Mukherjee (1990) suggested a number of flexible, floating barriers, sheet barriers and rigid floats to protect the cage structures from wave action. The floats dampen the wave thrust and absorb the wave energy before the wave can propagate and strike the cage and cause damage. Kumaraiah and Parameswaran (1985) proposed a circular cage that could be used in reservoir with moderate wave action for culture of carps, tilapia and air breathing fishes. The cages can float at the surface, remain just submerged or rest at the bottom. Floating cages are considered to be most appropriate for Indian conditions and all the experiments conducted so far in the country for seed rearing, growout, nutrition and biomonitoring have been in such enclosures.
Floating fish cages can be constructed out of a variety of materials including metal, wood, bamboo and netting. Fairly fine-meshed nylon netting is used for nursery purposes. Cages made of monofilament woven material of 1.0 to 3.0 mm mesh size are light and easy to handle but last only for six months to one year, depending on their thickness. Knotless nylon webbing of 3 to 6 mm mesh size and knotted nylon webbing of 7 to 15 mm mesh have been found to be very durable as cage material. A battery of cages can be buoyed up within a bamboo catwalk which will serve as a working platform, floated by sealed empty barrels. Circular and boxlike cages of varying diamensions on conduit pipe structures which can be easily assembled, and suitable flotation systems have been designed in India. Similarly, self–floating cage with HDPP pipe structure has also been experimented with succesfully.
In Jari tank near Allahabad, nylon cages (20 mesh cm-1; size 2.2×1.6×1.45 m) were stocked at a density of 8500 hatchlings m-2 (size 6.5 to 7.8 mm). These grew in 21 to 28 days to 30.2 to 45.6 mm with a survival of about 25% (Anon., 1979). In fry rearing, the stocking rate in the cages (mesh size 3 mm) was 700 to 2 500 m-2 and within 90 days they attained a size of 103.6 to 121.8 mm. The feed given was powdered soybean, groundnut cake and rice bran in equal proportions. Rearing of carp fry was done in Getalsud reservoir, where they (10 to 31 mm in size) were stocked in 2.4 × 1.5 × 1.5 m cages at the rate of 300 to 700 m-2. The growth rate per month was 17, 25 and 20 mm in mrigal, catla and rohu respectively. The stock was fed with mustard and groundnut cake and rice bran in the ratio 3:1:1 at 30% of the body weight (bw) of the stock for 4 days and thereafter at 20% for the rest of the period. Summary of cage culture experiments conducted is presented in Table 1.13.
A series of cage culture trials have been reported from a 12 ha impoundment in Bangalore (Parameswaran, 1993). In an experiment conducted with monofilament cloth cages of size 10.5 m-2, common carp and silver carp fry were reared at a ratio of 40:1 at a stocking density of 225 m-2. Put ona diet of powered rice bran, defatted silkworm pupae, groundnut cake and soya flour in 12:5:2:1 ratio at 10 to 20% bw day-1, the survival obtained at the end of 4 months rearing was 97.5% in common carp and 88% in silver carp. The stock attained average final weight of 20 and 8.6 g respectively. However, experiments conducted on catla gave erratic results with survival rate varying from 9 to 71.4%. In another trial, 30 000 spawn obtained from cage grown common carp parents were reared in 4.5 m3 auto-floating (PVC frame) monofilament cloth (mesh: 15 cm-1) cages. In 35 days, the fry attained a size of 25.4 mm with 38% survival. Restocked in 3.5 m3, 8 mm mesh knotless nylon netting cages, at a density of 475 m-3, they grew to 54.8 mm/4.9 in 75 days with a survival rate of 88.5%.
In a cage culture experiment reported from Tamil Nadu (Parameswaran, 1993), 10 days old fry (size 10 mm) stocked at a density of 500 m-2 were raised to a size of 50 to 60 mm in 40 days, with survival rates ranging from 45 to 85%. Department of Fisheries in Tamil Nadu has been undertaking rearing of spawn and fry of major carps in floating cages during July to September every year. However, data on the stocking density, nutrition, growth and survival are not available.
Rearing of the fry of Indian major carps was tried in Tungabhadra reservoirs in the year 1984–85 (Singit et al., 1985). Four floating cages, made of 16-P velon screen fitted on rectangular bamboo frame of 10×4×1 m, were stocked with rohu and mrigal. Although survival rates ranging from 37.5 to 87.5% were obtained at the end of the 3 months rearing period, the experiment was vitiated due to the destruction of cages due to heavy winds. At stocking densities ranging from 2 to 5 million ha-1, growth of about 100 mm (33 g) was obtained.
Dependent on the type of management input, fish production rates obtained for growout in cages vary greatly. Unlike the hi–tech system of saturated stocking and feeding on enriched formulated diets, the production recorded in cage culture of common carp is 35, 37.5 and 25 kg m-3 month-1 respectively in Japan, Germany and the Netherlands. In Asia, in general, only semi–intensive and low cost technologies are adopted, mainly due to economic considerations. In India, the growing season is almost year round, except for December–January in northern parts, where the temperature is low during these winter months.
|Species cultured||Cage volume (m3)||Stocking||Mean harvest size (g)||Culture period (months)||Production (kg m-2month-1)||Feed||Feeding rate (%bw||FCR||Reference|
|density (m-2)||size (g)|
|Cyprinus carpio||15.75||30–38||40–50||325||6||1.55–2.22S||WP, GNC, RB (8:9:3)||10–20||8.3–10.4||Govind(MS.) 1983|
|Catla catla||15.0–15.75||13–49||8–50||544–772||6–8||0.83–1.30||GNC,RB(1:1)||5–10||5.6–6.6||Govindet al., 1988|
|Hypoph- thalmichthys molitrix||10||15||61||472||10||0.7||SWP,RB, GNC (1:2:3)||3–5||3.1||Kumaraiahet al., 1991|
|Labeo calbasu||10||5||16.5||208||8||0.1||GNC, RB(1:1)||2||2.9||Kumaraiah et al., (unpublished)|
|Ctenopharyngodon idella||3||33–67||7–10||350–400||6||2.0–3.3||Lemna, Hydrilla||80||-||Bandopadhyay et al., 1991|
|Oreochromis mossambicus||5–10||100–200||6.0–7.6||32–62||2–5||09–1.6||RB, GNC, CFP (1:1:1)||3–5||1.8–2.3||Kumaraiah et al.,1986|
|Channa marulius||5||40||25.8||177||5.3||0.8||Trash fish||10–12||2.5||Kumaraiah Parameswaran; and (unpublished)|
|Clarias batrachus||2||100||7.4||36.9||3||1||-||-||-||Murugesan and Kumaraiah; 1972|
GNC= Groundnut cake;
CFP=cattle feed pellets;
FCRfood conservation ratio
Pen culture has a special relevance in reservoir management, since it has been widely recognised as a means to rear, in situ, the fingerlings for stocking. The number of fingerlings required for stocking the reservoirs in the country is so enormous that it is impossible to raise all of them in land-based nursery farms which makes pen nurseries sine qua non for reservoir management. Nevertheless, pen culture on a regular basis has not been practised anywhere in India except at Tungabhadra reservoir. The factors that hamper the standardisation of pen culture technique are:
the steep level fluctuations,
wind and wave action,
lack of suitable pen materials,
weed infestation and the related harvesting problems, and
nonsynchronisation of suitable water levels and the spawn availability.
The water retention time is important, since the rearing has to be completed before the water level in the pen goes down the critical limit. In reservoirs with high drawdown, the water retention time is very limited. Sometimes the filling takes place so late that no spawn of desirable carps will be available when the water level attains the desirable limit. The pen walls limiting the water circulation to some extent, the accumulated feed and fertilizers case eutrophication leading to weed infestation fouling of water and fish kills.
Pen culture in Tungabhadra reservoir
Despite all the limitations, pen nurseries are used with remarkable success in Tungabhadra reservoir for the last 12 years. During 1992–93, 21 pens were erected in Ladakanabhavi, 25 km away from the dam site, covering a total enclosure of 3.3 ha. The pen site is situated at an elevation of 496 m above MSL and the installation was completed in the month of July, when the site was still exposed. Later, when the water level increased, the pen got inundated.
The pen area was pre-treated with organic manure that resulted in a rich growth of plankton after the filling. A total of 15 million spawn were stocked in the pens, comprising 6.75 million Labeo rohita, and 8.25 million Cirrhinus mrigala. After a rearing period of 90 days, 2. 41 million fingerlings were collected from the pen and released into the reservoir. This included 1. 085 million L. rohita and 1. 325 million C. mrigala, worth Rs. 495 875. Pen culture operations on similar lines are being in Kyrdemkulai and Nongmahir reservoirs of the northeast (see Chapter on the Northeast).
Seed rearing experiments were conducted in a split bamboo pen enclosure of 247 .5 m2 reinforced with a nylon netting in Punjar swamp, adjoining the Bhavanisagar reservoir (Abraham, 1980a). The pen was stocked with the spawn of C. mrigala (size 7 mm) and L. fimbriatus (size 5 mm) at the rate of 4.6 million ha-1 and usual farm practices were followed. In 30 days, mrigal attained a size of 38 mm and L. fimbriatus, 28 mm. At the time of conclusion of the study after 3 months, the former had attained a size of 88 mm and the latter, 75 mm. The overall survival obtained was 27.8%.
A pen culture experiment for raising catla and rohu in Manika maun, a floodplan lake in Gandak basin yielded a (computed) production of 4 t fish ha-1 in six months. The experiment was conducted in a bamboo screen pen (1000 m2) and the stock was fed with mixture of rice bran and mustard cake, apart from a feed formulated from the aquatic the weeds collected from the lake.
The presence of underwater obstacles restricts the use of active gear in reservoirs and the choice is often limited to passive gear such as simple gill nets. The most common among them is the Rangoon net, an entangling type of gill net without a foot rope. Another entangling type of net used in reservoirs is uduvalai, which has a reduced fishing height and is usually operated in shallow marginal areas to catch small fish. Shore seines of various dimensions and mesh sizes are employed in many reservoirs. Although a number of other fishing gear such as long lines, hand lines, pole and line, cast nets, dip nets, etc. are in use, their contribution to the total catch is very insignificant.
Unlike the marine fisheries, very little attention has been paid over the years towards improvement of gear in the inland sector, barring an attempt to upgrade the reservoir fishing gear by two experts under the aegis of FAO/UNDP programme during the 1960s (Gulbadamov, 1962). A number of improvements have been suggested by the experts to the fishing techniques followed by the reservoir fishermen. Apart from the introduction of frame net, they have suggested improvements on the design of gill nets, beach seines and long lines, Unconventional methods such as electrical fishing, use of light as fish lure, and the use of echosounder for fish detection and survey of bottom topography were also suggested.
The main emphasis of gear improvement was the modification of gill nets. Gulbadamov (1962) designed two nets viz., Sebgul I and Sebgul II, which were gill nets with modified rigging patttern. While ensuring a proper fixing of webbing on head and foot ropes, a uniform hanging coefficient was ensured. Similarly, the sideways movement of the webbing was checked to maintain effective area of the net. Ranganathan and Venkataswamy (1967) conducted experiments in Bhavanisagar reservoir to check the efficacy of the new design and found no appreciable superiority for Sebgul nets over the Rangoon nets.
The Central Institute of Fisheries Technology has experimented with gill nets of various colours and found yellow and orange coloured nets yielding better catch than the white coloured ones(Table 1.14). It has been observed that 77% of the carps were caught by entangling and the rest by gilling. The usual method of increasing the entangling capacity of gill net is by decreasing the slackness of webbing, which can be achieved by suitable modifications in the hanging of the net (Nayar, 1979).
Alivi, the giant drag net of Tungabhadra reservoir is described in the chapter on Karnataka. This giant shore seine catch fishes of all hues in large numbers, including the juveniles of commercially important carp species. Similar nets are used in Rihand and Keetham reservoirs, where they are known as mahajal.
|Colour of net||Catch kg 1000 m-2|
(After Nayar, 1979)
Trawling has been attempted only in two reservoirs viz., Gandhisagar and Hirakud. The findings of Kartha and Rao (1990) with regard to species selectivity of different type of trawling are interesting. The findings suggest efficacy of various types of trawling in increasing the productivity of commercial carps and checking the predator and weed fish populations. After experimental trawling with single-boat bottom trawling, two-boat bottom trawling and two-boat mid-water trawling under different speeds, it has been found that more than 92% of the total catch consisted of economic varieties such as catla, rohu, murrels, mullets and featherbacks in two-boat mid-water trawling. This was in sharp contrast to the bottom trawling, of both single boat and two boat variety, which yielded mostly (64 to 91%) the noncommercial species of fish. The two-boat mid-water trawling at a speed of 3 to 4 knots have been recommended for exploitation of commercial species and single and two-boat bottom trawling at 2 to 3 knots for eradication of uneconomic species of fishes.
Coracle, a saucer shaped country craft, is the major fishing craft used in the reservoirs of peninsular India. It is made of a split bamboo frame, covered with buffalo hide. Apart from being simple and inexpensive, coracle is durable and has very good manoeuvrability in choppy waters. It is also a versatile craft used for laying and lifting of nets, besides navigation and transport of fish and other material. Coracles of Krishnarajasagar are prepared by wrapping HDPP over the bamboo frame with the help of coal tar as an external covering in place of hide. This modified version of coracle is cheaper and more durable (Anon., 1984b; Parameswaran and Murugesan, 1984).
Unlike Gobindsagar, where all the fishermen possess their boats, reservoir fishermen, in general, are too poor to own boats. In many reservoirs like Vallabhsagar and Hirakud, the fishermen could get their boats with the help of subsidy and other financial assistance from the Government or fundings agencies. In Vallabhsagar, boats are distributed by the State among the fishermen at a subsidy of 50%, while in Hirakud, they get it from various schemes under NABARD and NCDC. Wooden boats are used for fishing in a number of reservoirs, especially in the North India. Flat bottomed, locally fabricated boats ranging in length from 3 to 7m are used in Kyrdemkulai, Hirakud, Malampuzha, Gobindsagar, Gandhisagar and Rihand. A plank-built, flat bottomed canoe, 2 to 3 m in length is the most popular fishing craft of Gandhisagar. In the same reservoir the repatriates from the erstwhile East Pakistan used the Bengal type dinghy, which is 5 to 7 m in length and have the additional facility of setting sails for wind propulsion.
Mechanised boats are not used in reservoir fishing in any appreciable extent. A 9.1 m long wooden, mechanised boat has been introduced by the CIFT in Hirakud reservoir, but they are too expensive for the fishermen. It is significant to note that large water bodies like Nagarjunasagar, Tungabhadra and Krishnarajasagar have no motorised craft neither for fishing nor for fish transport.
Dugout canoes, carved out of palm trees are used in Yerrakalava reservoir. In most of the reservoirs in the country the fishermen rely on improvised materials. Reservoir fishermen show considerable ingenuinity in fabricating makeshift rafts out of discarded old tyres, logs, used cans etc. In a vast majority of Indian reservoirs, where the catch is not very remunerative, no boats are used and the fishermen depend entirely on these improvised devices.
Until recently, pre-impoundment surveys conducted in India in connection with dam construction invariably lacked a fisheries perspective. Faunistic surveys of the river stretches carried out before dam construction in Tungabhadra (Chacko and Kuriyan, 1948), Bhavanisagar (Chacko and Dinamani, 1949), Pipri and Rihand (Hora, 1949), Damodar Valley Corporation reservoirs (Job et al.,1952), Hirakud (Job et al., 1955) and Gandhisagar (Debey and Mehra, 1959) were not comprehensive and did not help in any way the conservation and developmental efforts. A complete survey comprising the fish and fisheries, together with inventories of fishing villages and fishermen populations, and fishing craft gear for planned development is lacking in most cases.
The pre-impoundment surveys provide the framework for future development policies and should encompass:
the native ichthyofauna in the river stretch above and below the dam, and their likely chances of survival,
breeding habits of fishes and the possible impact of impoundment on their recruitment.
survey of breeding grounds in relation to submergence, both above and below the dam,
hydrobiological characteristics of water and soil with special emphasis on the nutrient and thermal regime,
needs for creating infrastructure such as, hatcheries, nurseries, ice plants etc. ,
site selection for pen nurseries, cages etc., and
possibilities for cleaning the area of submergence of trees and other obstructions.
Holistic pre-impoundment survey for fisheries development is a new concept in India. A beginning in this direction has been made by the Narmada Control Authority. A recently concluded socio-economic survey of the Narmada basin tried to address the problem of fisheries development with a holistic approach.
Opinion is divided on the wisdom of removing timber from the reservoir bed. While it is mostly appreciated that a reservoir bed free from obstructions facilitates the use of active fishing gear and leaves room for many other management options, many workers feel the necessity to leave at least the non-commercial timber intact for a variety of purposes such as, reducing wave action, flocculating the colloidal clay turbidity, providing habitat for fishes and substrata for periphyton deposition (Bhukaswan, 1980). Timber clearance has been tried in a number of reservoirs in India, both before and after the impoundment. In Chillar and Benisagar reservoirs of Madhya Pradesh, trees were cut from the lake bed and auctioned before the reservoirs were filled. Harsi, Jamoia and Ghatera reservoirs are examples of complete clearance of date palm trees from the marginal areas during the summer months. Forest area of about 61. 4 km2 were cleared in Hirakud, when the bed was exposed during drawdown.
Fisheries being a state subject, management of reservoir fisheries vests with the State Governments. There is a great deal of divergence in the management practices followed by individual States which vary from outright auctioning to almost free-fishing. Cooperative societies (primary and apex) and the State level Fisheries Development Corporations are also involved in the fishing and marketing operations. Involvement of the above agencies and their role in fishery operations and market interventions often vary from one reservoir to another within the same State. Some sort of uniformity in fishery regulations among various categories of reservoirs as well as the need to monitor the socio-economic aspects of reservoirs fisheries.
Commercial exploitation systems followed in different States can be broadly classified under four headings viz., 1) departmental fishing, 2) lease by auctioning, 3) issue of licences to cooperative societies or individuals and 4) fishing on a royalty basis (crop sharing). Direct departmental fishing being not an economic proposition, is followed only in a very few reservoirs. In some reservoirs like Hirakud, Nagarjunasagar, and DVC, this practice is partially resorted to for experimental and exploratory purposes. In most of the cases, the Department exerts its control over the exploitation by acting as a marketing link and controlling the fishing effort. In Rajasthan, Madhya Pradesh and Uttar Pradesh, the small reservoirs are mostly auctioned on an yearly basis. In a number of large reservoirs, free licences are issued to fishermen without any limit. This virtual free for all system has been found to be detrimental to the interests of the ecosystem and fishermen in Nagarjunasagar, Yerrakalava and a large number of other reservoris in Andhra Pradesh. Crop sharing is a very popular mode of exploitation in Tamil Nadu, where, the fishermen are provided with all fishing implements, in return of which they pay a royalty (sometimes up to 50% of the catch) to the Government.
Evaluation of the socio-economics of reservoir fisheries is a very tedious task due to the multiplicity of agencies involved in reservoir management. Reservoir fisheries is developed basically on capture fisheries lines, following the common property norm. Like the rivers, lakes and the seas, the biological wealth is considered as a nature's endowment and the State's intervention in developmental activities benefits the poor fishermen who toil in water. The investment made in developing reservoir fisheries shall be viewed in the light of the social benefits it accrues in the form of:
rehabilitating the displaced population,
improving the living conditions of fishermen, and
providing employment opportunities.
Capture fisheries activity of the reservoirs is akin to the extractive industries like coal, oil, iron ore etc., where the yield depends on the state of technology involved and the quantum of labour and capital deployed. But the renewable nature of the resource and the intricate biological principles involved in the ecosystem management imparts a heavy element of challenge into the reservoir fisheries management. Human intervention being less intense in reservoirs, compared to the aquaculture operations, yields often display violent fluctuations, even if the effort in terms of labour and capital is kept constant. Therefore, production-function relationship is bound to be intricate and less precise. A certain measure of stability needs to be imparted in production by affecting sustained improvement in yield along with remunerative returns to fishermen by narrowing the price spread between the producer and the consumer.
In aquaculture, it is estimated that 77.23% of the price paid by the consumer is received by the producer (Paul, 1990). As opposed to this, a major chunk of the price is siphoned away by the wholesalers and other market intermediaries in reservoir fisheries. A study of seven reservoirs (Paul and Sugunan, 1990) for a period of six years has brought to light the major factors that determine the remunerativeness of fishing in reservoirs. Reservoir fisheries is a sector, where the chief input is labour, besides a marginal depreciation of crafts and gear. Even if the costs of stocking and other developmental measures are taken into account, this area does not call for heavy investment as in the case of pond culture.
A factor that can bring serious distortions in the income level of fishermen is the over–concentration of fishermen in the wake of low fish productivity. For instance, in Ukai reservoir, with an area of 36 525 ha, 306 boats with 3 400 gill nets (50 m each) were operated during 1985–86 to 1982–83. In a fishing year comprising 260 days, 1 836 fishermen netted out 174 t of fishes. In sharp contrast to this, 520 fishermen of Nagarjunasagar shared a catch of 170 t in a year. In lower Aliyar, 17 t of fishes were harvested by 14 fishermen, each of them, after meeting the royalty obligations, could take home only Rs. 1 000 to 1 400 a year. A better picture, however, emerged in Bhavanisagar, where 80 fishermen after sharing 150 to 300 t of fishes, earned an annual individual income of Rs. 8 175/ (Paul and Sugunan, 1983).
There is a need to dovetail the twin objectives of conservation and yield optimisation in reservoir fisheries management. While the fishermen and the fish merchants strive to increase the production for economic considerations, it is the responsibility of the State to ensure that economic expediency of development does not mar the ecological reasoning. Virtual free fishing, as followed in Andhra Pradesh is counter–productive to the norms of conservation and yield optimisation. Although there are fair possibilities of linking reservoir fisheries development with poverty alleviation programmes, the progress made so far in this direction is not very encouraging. Chances of creating additional employment are not much in majority of Indian reservoirs. On the contrary, many reservoirs have surplus manpower which can be diverted to others which can absorb more men without eroding the income level of the existing fishermen (Paul and Sugunan, 1990).
Figure 2.1. Distribution of reservoir and tanks in Tamil Nadu